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Exclusive look inside the Niger ambush zone 01:53

Washington (CNN)The Department of Defense has said it expects the investigation into the October 4 ambush in Niger that killed four US soldiers to be completed by January, according to a statement released Wednesday.

The Pentagon has updated the families of the soldiers killed on the timeline of the investigation.

"Families were informed that AfriCom's investigation team will travel to locations in the US, Africa and Europe to gather information related to the investigation," the statement said. "US Army Maj. Gen. Roger Cloutier, AfriCom's chief of staff, will lead the command's investigation team."

The statement added that the families had been told that the "investigation is expected to be completed in January 2018, but that if circumstances require additional time, that they will be kept informed."

"The Department of Defense is committed to a thorough and detailed investigation into the deaths of Staff Sgt. Bryan C. Black, Staff Sgt. Jeremiah W. Johnson, Staff Sgt. Dustin M. Wright, and Sgt. La David T. Johnson. Once the investigation is completed, briefing the families of the fallen will be the Department's first priority," the statement also said.

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Outnumbered and lightly armed, a Green Beret-led team that was ambushed by militants in Niger last month became separated during the firefight, US officials and a Nigerien soldier told CNN.

The squad was traveling with 30 Nigerien soldiers when they were attacked by approximately 50 ISIS-affiliated fighters armed with rocket propelled grenades, mortars, and heavy machine guns, according to US military officials.

During the subsequent gun battle, which lasted for hours, four US soldiers were killed and two were wounded. Five Nigeriens were also killed. US-Nigerien forces managed to kill 20 militants during the firefight according to a defense official.

The body of one American soldier, Sgt. La David Johnson, was recovered 48 hours after the attack.